Method for driving an electronic switch in a power converter circuit and control circuit

ABSTRACT

A method and a control circuit for driving an electronic switch coupled to an inductor in a power converter in successive drive cycles each including an on-time and an off-time are disclosed. Driving the electronic switch includes: measuring an inductor voltage during the on-time in a drive cycle in order to obtain a first measurement value; measuring the inductor voltage during the off-time in a drive cycle in order to obtain a second measurement value; obtaining a first voltage measurement signal that is dependent on a sum of the first measurement value and the second measurement value; and adjusting the on-time in a successive drive cycle dependent on a feedback signal and the first voltage measurement signal.

RELATED APPLICATIONS

This application is a continuation application of earlier filed U.S.patent application Ser. No. 16/902,812 entitled “METHOD FOR DRIVING ANELECTRONIC SWITCH IN A POWER CONVERTER CIRCUIT AND CONTROL CIRCUIT,”filed on Jun. 16, 2020, the entire teachings of which are incorporatedherein by this reference.

U.S. patent application Ser. No. 16/902,812 claims priority to earlierfiled European Patent Application Serial Number EP19183979 entitled“METHOD FOR DRIVING AN ELECTRONIC SWITCH IN A POWER CONVERTER CIRCUITAND CONTROL CIRCUIT,” filed on Jul. 2, 2019, the entire teachings ofwhich are incorporated herein by this reference.

BACKGROUND

Switched-mode power converter circuits are widely used to convert powerin various kinds of electronic applications such as automotive,industrial, telecommunication, household or consumer electronicapplications. A switched-mode power converter may include an electronicswitch, an inductor coupled to the electronic switch, and a rectifiercircuit coupled to the inductor. Converting power with a switched-modepower converter of this type usually includes receiving an input voltageand an input current at an input by the power converter and driving theelectronic switch in a plurality of successive drive cycles eachincluding an on-time and an off-time, wherein the inductor receivesenergy from the input during the on-time and transfers energy to therectifier circuit during the off-time. An output parameter of the powerconverter, such as an output voltage or an output current, may beregulated by suitably adjusting durations of the on-times and theoff-times.

BRIEF DESCRIPTION OF EMBODIMENTS

Some types of power converter circuits are configured to receive, as theinput voltage, an alternating voltage or a rectified alternating voltageand, in addition to regulating the output parameter, are configured toregulate an input current such that an average input current in theindividual drive cycles is essentially proportional to the inputvoltage. A power converter circuit of this type is usually referred toas PFC (Power Factor Correction) power converter, PFC converter, or PFCstage. A drive circuit configured to drive the electronic switch in aPFC converter is usually referred to as PFC controller.

There is a need for a simple and efficient method for driving anelectronic switch in a PFC power converter, in particular a PFCconverter in which input and output are galvanically isolated, and adrive circuit configured to implement this method.

One example relates to a method. The method includes driving anelectronic switch coupled to an inductor in a power converter insuccessive drive cycles each including an on-time and an off-time,wherein the inductor is coupled to an auxiliary winding. Driving theelectronic switch includes measuring an auxiliary voltage across theauxiliary winding during the on-time in a drive cycle in order to obtaina first measurement value, measuring the auxiliary voltage during theoff-time in a drive cycle in order to obtain a second measurement value,obtaining a first voltage measurement signal that is dependent on a sumof the first measurement value and the second measurement value, andadjusting the on-time in a successive drive cycle dependent on afeedback signal and the first voltage measurement signal.

Another example relates to a control circuit configured to drive anelectronic switch coupled to an inductor in a power converter insuccessive drive cycles, each including an on-time and an off-time. Thecontrol circuit is configured to measure an auxiliary voltage across anauxiliary winding coupled to the inductor during the on-time of a drivecycle in order to obtain a first measurement value, to measure theauxiliary voltage during the off-time of a drive cycle in order toobtain a second measurement value, to obtain a first voltage measurementsignal that is dependent on a sum of the first measurement value and thesecond measurement value, and to adjust the on-time in a second drivecycle dependent on a feedback signal and the first voltage measurementsignal.

Examples are explained below with reference to the drawings. Thedrawings serve to illustrate certain principles, so that only aspectsnecessary for understanding these principles are illustrated. Thedrawings are not to scale. In the drawings the same reference charactersdenote like features.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates one example of a power converter;

FIG. 2 illustrates one example of a rectifier circuit in the powerconverter according to FIG. 1 ;

FIG. 3 shows signal diagrams that illustrate operating the powerconverter in a first operating mode;

FIG. 4 illustrates one example of an input voltage that may be receivedby the power converter according to FIG. 1 ;

FIG. 5 is a flowchart that illustrates operating the power converter inthe first operating mode;

FIG. 6 illustrates one example of a control circuit configured tocontrol operation of the power converter and drive an electronic switchin the power converter;

FIG. 7 illustrates one example of a drive circuit in the control circuitshown in FIG. 6 ;

FIG. 8 illustrates one example of an on-time control circuit in thedrive circuit;

FIG. 9 shows signal diagrams that illustrate the function of the on-timecontrol circuit shown in FIG. 8 ;

FIG. 10 illustrates another example of an on-time control circuit;

FIG. 11 illustrates yet another example of an on-time control circuit;

FIG. 12 illustrates one example of a ramp signal generator;

FIG. 13 show signal diagrams that illustrate the functionality offunction generators in the on-time control circuit according to FIG. 11;

FIG. 14 shows one example of a voltage measurement circuit;

FIG. 15 shows another example of the voltage measurement circuit;

FIGS. 16A and 16B illustrate one example of the voltage measurementcircuit shown in FIG. 15 in greater detail;

FIG. 17 shows signal diagrams that illustrate operating the powerconverter in a second operating mode;

FIG. 18 is a flowchart that illustrates operating the power converter inthe second operating mode;

FIG. 19 illustrates a modification of the control circuit shown in FIG.6 ;

FIG. 20 illustrates a modification of the voltage measurement circuitshown in FIG. 14 ;

FIG. 21 illustrates a modification of the voltage measurement circuitshown in FIGS. 16A and 16B;

FIG. 22 illustrates one example of a drive circuit in the controlcircuit shown in FIG. 18 ;

FIG. 23 illustrates the functionality of a zero crossing controller inthe drive circuit shown in FIG. 22 ;

FIG. 24 illustrates one example of a further on-time control circuit inthe drive circuit shown in FIG. 22 ;

FIG. 25 illustrates one example of a ramp signal generator in thefurther on-time control circuit shown in FIG. 24 ;

FIG. 26 shows signal diagrams that illustrate the functionality of thecontrol circuit shown in FIG. 19 ;

FIG. 27 illustrates one example of a signal generator in the secondon-time control circuit shown in FIG. 24 ;

FIG. 28 shows a signal diagram that illustrates the functionality of thesignal generator shown in FIG. 27 ;

FIG. 29 illustrates a power converter according to another example; and

FIG. 30 shows signal diagrams that illustrate operating the powerconverter shown in FIG. 29 in a second operating mode.

In the following detailed description, reference is made to theaccompanying drawings. The drawings form a part of the description andfor the purpose of illustration show examples of how the invention maybe used and implemented. It is to be understood that the features of thevarious embodiments described herein may be combined with each other,unless specifically noted otherwise.

BRIEF DESCRIPTION AND DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates one example of a power converter. This powerconverter includes an input 11, 12 configured to receive an inputvoltage V_(IN) and an output 13, 14 configured to provide an outputvoltage V_(OUT) and an output current I_(OUT). The power converter maybe configured to regulate an output parameter such as the output voltageV_(OUT), the output current I_(OUT), or an output power (which is givenby the output voltage V_(OUT) multiplied by the output current I_(OUT))such that the output parameter has a predefined value.

Referring to FIG. 1 , the power converter further includes a switchingcircuit 2 connected to the input 11, 12 and a rectifier circuit 3connected between the switching circuit 2 and the output 13, 14. Theswitching circuit 2 includes an electronic switch 22 and an inductor 21coupled to the electronic switch 22. The electronic switch 22 iscontrolled by a drive signal S_(DRV) received at a drive input of theelectronic switch 22 such that the electronic switch 22 switches on oroff dependent on the drive signal S_(DRV). Any type of electronic switchsuch as a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor), anIGBT (Insulated Gate Bipolar Transistor), a HEMT (High Electron-MobilityTransistor), or the like may be used to implement the electronic switch22.

Just for the purpose of illustration, the power converter shown in FIG.1 is a flyback converter. In this case, the inductor 21 is a transformerwith a primary winding 21 ₁ and a secondary winding 21 ₂, wherein theprimary winding 21 ₁ is connected in series with the electronic switch22 and the series circuit including the primary winding 21 ₁ and theelectronic switch 22 is connected to the input 11, 12. That is, theseries circuit including the primary winding 21 ₁ and the electronicswitch 22 is connected between a first input node 11 and a second inputnode 12 of the input 11, 12. The secondary winding 21 ₂ is inductivelycoupled to the primary winding 21 ₁ and is connected to the rectifiercircuit 3. According to one example, a winding sense of the secondarywinding 21 ₂ is opposite to a winding sense of the primary winding 21 ₁.In the power converter shown in FIG. 1 , a current I2 through theprimary winding 21 ₁ of the transformer is controlled by a switched-modeoperation of the electronic switch 22. This is explained in furtherdetail herein below.

Optionally, the power converter further includes an auxiliary winding 23of the transformer. This auxiliary winding 23 is inductively coupled tothe primary winding 21 ₁ and the secondary winding 21 ₂. A voltageV_(AUX) across the auxiliary winding 23 is referred to as auxiliaryvoltage in the following. The auxiliary voltage V_(AUX) is essentiallyproportional to a voltage V21 ₁ across the primary winding 21 ₁, whereina proportionality factor between the auxiliary voltage V_(AUX) and thevoltage V21 ₁ across the primary winding 21 ₁ is dependent on a ratiobetween a number of turns of the auxiliary winding 23 and a number ofturns of the primary winding 21 ₁. The voltage V21 ₁ across the primarywinding is also referred to as inductor voltage or primary voltage inthe following. According to one example, the auxiliary voltage V_(AUX)is a voltage referenced to the second input node 12. This second inputnode 12 is also referred to as ground node GND in the following.

Referring to FIG. 1 , the power converter further includes a feedbackcircuit 15. The feedback circuit 15 receives an output signal S_(OUT)that represents the output parameter that is to be regulated. That is,the output signal S_(OUT) may represent the output voltage V_(OUT), theoutput current I_(OUT), or the output power P_(OUT). According to oneexample, the output signal S_(OUT) represents the output voltage V_(OUT)and is essentially proportional to the output voltage V_(OUT). Accordingto another example, the output signal S_(OUT) represents the outputcurrent I_(OUT) and is essentially proportional to the output currentI_(OUT). The output signal S_(OUT) may be obtained by measuring therespective output parameter in a conventional way using any kind ofvoltage, current or power measurement circuit. Such circuits arecommonly known so that no further explanations are required in thisregard.

The feedback circuit 15 is configured to generate a feedback signalS_(FB) based on the output signal S_(OUT). The power converter furtherincludes a drive circuit 4 that receives the feedback signal S_(FB) at afirst input 41 and is configured to generate the drive signal S_(DRV) atan output 40 based on the feedback signal S_(FB). Referring to FIG. 1 ,a load Z (illustrated in dashed lines) connected to the output 13, 14may receive the output power provided by the power converter. Dependenton a power consumption of the load Z, the regulated output parameter,such as the output voltage V_(OUT) or the output current I_(OUT), mayvary. The feedback circuit 15 is configured to generate the feedbacksignal S_(FB) in such a way that the feedback signal S_(FB)—based onwhich the electronic switch 22 is driven—counteracts such variations ofthe regulated output parameter. More specifically, the feedback signalS_(FB) is generated in such a way that, in a steady state of the powerconverter, an input power of the power converter, which is controlled bythe switched-mode operation of the electronic switch 22, essentiallyequals the output power received by the load Z. The feedback circuit 15and the drive circuit 4 form a control loop that is configured toregulate the output parameter.

Generating the feedback signal S_(FB) by the feedback circuit 15 mayinclude comparing the output signal S_(OUT) with a reference signalS_(REF), calculating an error signal based on comparing the outputsignal S_(OUT) with the reference signal S_(REF), and generating thefeedback signal S_(FB) based on the error signal. The reference signalS_(REF) represents a desired value of the output parameter. Generatingthe feedback signal S_(FB) based on the error signal may includefiltering the error signal using a filter with any one of a P(proportional) characteristic, an I (integrating) characteristic, a P1(proportional-integrating) characteristic or a PID(proportional-integrating-deriving) characteristic. Generating afeedback signal in a power converter based on an error signal iscommonly known, so that no further explanation is required in thisregard.

Referring to FIG. 1 , a coupler 16 may be connected between the feedbackcircuit 15 and the drive circuit 4. Due to the transformer 21 there is agalvanic isolation between the input 11, 12 and the output 13, 14. Thecoupler 16 is configured to transmit the feedback signal S_(FB) from thefeedback circuit 51 via the galvanic isolation to the drive circuit 4.The coupler 16 may include an optocoupler, an inductive coupler, acapacitive coupler, or the like. In the example shown in FIG. 1 , thecoupler 16 is connected between the feedback circuit 15 and the drivecircuit 4. In this case, the feedback circuit 15 is arranged on asecondary side of the power converter. This, however, is only anexample. According to another example (not shown), the feedback circuitis arranged on the primary side and receives the auxiliary voltageV_(AUX). The auxiliary voltage V_(AUX) is essentially proportional tothe output voltage V_(OUT) when the switch is in the off-state. Thelatter is explained with reference to FIG. 3 herein below.

The rectifier circuit 3 is configured to rectify a voltage V21 ₂ acrossthe secondary winding 21 ₂. The rectifier circuit 3 may be implementedin various ways. One example of the rectifier circuit 3 is illustratedin FIG. 2 . In this example, the rectifier circuit 3 includes a seriescircuit with a rectifier element 31 and a capacitor 32 connected to thesecondary winding 21 ₂. The output voltage V_(OUT) is a voltage acrossthe capacitor 32 according to one example. The load Z that may beconnected to the load may be any kind of load or load circuit. The loadZ may include a further power converter that is configured to provide avoltage with a voltage level different from the voltage level of theoutput voltage V_(OUT).

Operating the electronic switch 22 in a switched-mode fashion includesoperating the electronic switch 22 in a plurality of successive drivecycles, wherein in each of these drive cycles the electronic switch 22switches on for an on-time and switches off for an off-time. This isexplained in further detail with reference to FIG. 3 below.

FIG. 3 illustrates signal diagrams of the current I2 through the primarywinding 21 ₁ and the switch 22, a current I3 through the secondarywinding 21 ₂, a voltage V21 ₁ across the primary winding 21 ₁, theauxiliary voltage V_(AUX), a voltage V22 across the switch 22, and thedrive signal S_(DRV). In the following, the current I2 through theprimary winding 21 ₁ and the switch 22 is also referred to as primarycurrent, the current I3 through the secondary winding 21 ₂ is alsoreferred to as secondary current, the voltage V21 ₁ across the primarywinding 21 ₁ is also referred to as primary voltage, and the voltage V22across the switch 22 is also referred to as switch voltage. FIG. 3illustrates operating the power converter in one drive cycle. A durationT of this drive cycle is given by a duration TON of an on-time plus aduration T_(OFF) of an off-time. The “on-time” is the time period inwhich the electronic switch 22 is switched on, and the “off-time is thetime period in which the electronic switch 22 is switched off. Theelectronic switch 22 switches on when the drive signal S_(DRV) has anon-level and switches off when the drive signal S_(DRV) has anoff-level. Just for the purpose of illustration, the on-level is a highsignal level and the off-level is a low signal level in the exampleillustrated in FIG. 3 .

Referring to FIG. 3 , the primary current I2 increases during theon-time (wherein an increase of the primary current I2 is essentiallyproportional to the input voltage V_(IN) and inversely proportional toan inductance of the transformer 21). Further, during the on-time, thesecondary current I3 is zero, the primary voltage V21 ₁ essentiallyequals the input voltage V_(IN), and the switch voltage V22 isessentially zero. In the example illustrated in FIG. 1 , the windingsense of the auxiliary voltage V_(AUX) is such that the auxiliaryvoltage V_(AUX) is negative during the on-time. Referring to the above,a magnitude of the auxiliary voltage V_(AUX) is proportional to amagnitude of the primary voltage V21 ₁. Thus, during the on-time, amagnitude V1 of the auxiliary voltage V_(AUX) is proportional to theinput voltage V_(IN).

Referring to FIG. 3 , when the switch 22 switches off, the primarycurrent I2 becomes zero and the secondary current I3 jumps to an initialvalue from which it gradually decreases. The primary voltage V21 ₁ and,equivalently, the auxiliary voltage V_(AUX) change their polarity. Themagnitude of the primary voltage V21 ₁ is essentially given byn·V_(OUT), wherein n is given by a ratio between a number N₁ of turns ofthe primary winding 21 ₁ and a number of turns N₂ of the secondarywinding 21 ₂ (n=N₁/N₂). During the off-time, a magnitude V2 of theauxiliary voltage V_(AUX) is again proportional to the primary voltageV21 ₁, so that during the off-time the magnitude V2 of the auxiliaryvoltage V_(AUX) is proportional to the output voltage V_(OUT). Theproportionality factor between the auxiliary voltage V_(AUX) and theprimary voltage V21 ₁ is the same during the on-time and during theoff-time. Further, during the off-time, the switch voltage V22essentially equals the input voltage V_(IN) plus the magnitude of theprimary voltage V21 ₁. It should be noted that the primary voltage V21 ₁is not exactly proportional to the output voltage V_(OUT), but isproportional to the output voltage V_(OUT) plus a voltage across therectifier circuit 3, wherein the voltage across the rectifier circuit 3decreases as the transformer is demagnetized. This voltage across therectifier circuit 3, however, is negligible as compared to the outputvoltage V_(OUT), so that the primary voltage V21 ₁ can be considered tobe proportional to the output voltage V_(OUT) during the off-time.

During the on-time, energy is magnetically stored in the transformer 21and, during the off-time, this energy is transferred from thetransformer 21 via the rectifier circuit 3 to the output 13, 14. Storingenergy in the transformer 21 is associated with magnetizing thetransformer and transferring the energy from the transformer 21 to theoutput 13, 14 is associated with demagnetizing the transformer. In theexample illustrated in FIG. 3 , the off-time is long enough for thetransformer 21 to be completely demagnetized. When the transformer iscompletely demagnetized, the (negative) primary voltage V21 ₁ increasesand the (positive) auxiliary voltage V_(AUX) decreases. In the exampleshown in FIG. 3 , the electronic switch 22 again switches on when theauxiliary voltage V_(AUX) crosses zero or shortly after the auxiliaryvoltage V_(AUX) crosses zero. This kind of operating mode of the powerconverter is referred to as first operating mode or quasi-resonant modein the following. In this type of operating mode the duration T of onedrive cycle is dependent on the duration TON of the on-time and theduration T_(OFF) of the off-time, wherein the duration of the off-timeincreases as the duration of the on-time increases and wherein, for agiven duration of the on-time, the duration of the off-time increases asthe input voltage V_(IN) increases.

In FIG. 3 , t_(DEMAG) denotes a demagnetization time instance, which isa time instance when the transformer 21 has been completelydemagnetized. The proportionality explained above between the primaryvoltage V21 ₁ and the output voltage V_(OUT) is given in a time periodT_(DEM) between the beginning of the off-time and the demagnetizationtime instance t_(DEMAG). This time period T_(DEM) is also referred to asdemagnetization time period in the following.

According to one example, the input voltage V_(IN) is a rectifiedalternating voltage such as a rectified sinusoidal voltage illustratedin FIG. 4 . A rectified sinusoidal voltage may be generated by a bridgerectifier (not illustrated in the drawings) from a sinusoidal gridvoltage, for example.

In many different applications of a power converter of the type shown inFIG. 1 it is desired not only to control the output parameter such thatit essentially equals a desired value, but it is also to control theinput current I_(IN), which is the current I2 through the primarywinding 21 ₂ and the switch 22 in the example shown in FIG. 1 , suchthat an average of the input current I_(IN) follows the signal waveformof the input voltage V_(IN). That is, it is desirable to control theinput current I_(IN) such that in each drive cycle the average of theinput current I_(IN) is essentially proportional to an instantaneousvalue of the input voltage V_(IN), wherein a proportionality factorbetween the average of the input current I_(IN) and the input voltageV_(IN) may vary dependent on the power consumption of the load Z. Apower converter having this functionality is often referred to as PFC(Power Factor Correction) converter. The average of the input currentI_(IN) is also referred to as average input current I_(IN_AVG) in thefollowing.

It should be noted that the power received by the power converter fromthe input is proportional to a square of the input voltage. When theinput voltage V_(IN) is a sinusoidal voltage, for example, the inputpower received by the power converter has a sine square waveform,wherein an amplitude of this sine square waveform is dependent on apower consumption of the load. At least one capacitor of the rectifiercircuit 3, such as capacitor 32 in FIG. 2 , ensures that an essentiallyconstant output voltage V_(OUT) and an essentially constant outputcurrent I_(OUT) can be provided. Nevertheless, the output voltageV_(OUT) might not be exactly constant but may include a periodic voltageripple with a frequency that is given by twice the frequency of theinput voltage V_(IN). This voltage ripple is due to the variation of theinput power that occurs when the average input current I_(IN_AVG) isproportional to the input voltage V_(IN). The feedback circuit 15 may beconfigured to filter out these ripples so that they do not negativelyaffect the control loop. Filtering out these ripples may include using anotch filter or using an integrating filter with a relatively long anintegrating window.

FIG. 5 shows a flowchart of a method that is configured to controloperation of the power converter such that both regulating the outputparameter and regulating the waveform of the average input currentI_(IN_AVG) is achieved in a rather simple way. Referring to FIG. 5 , themethod includes measuring the inductor voltage during the on-time in onedrive cycle in order to obtain a first measurement value (101),measuring the inductor voltage during the off-time in one drive cycle inorder to obtain a second measurement value (102), and adjusting aduration of the on-time in at least one subsequent drive cycle based ona sum of the first measurement value and the second measurement valueand based on the feedback signal (103). The inductor voltage is thevoltage V21 ₁ across the primary winding, that is the primary voltageaccording to one example. According to one example, obtaining the firstmeasurement value and the second measurement value includes obtainingthe first measurement value such that it is proportional to the inductorvoltage during the on-time and obtaining the second measurement valuesuch that it is proportional to the magnitude of the inductor voltageduring the off-time.

According to one example, the inductor voltage is either the primaryvoltage V21 ₁ or the secondary voltage V21 ₂ and measuring the inductorvoltage V21 ₁ includes measuring that auxiliary voltage V_(AUX) which,as explained above, is proportional to the primary voltage V21 ₁ (andthe secondary voltage V21 ₂). In this case, obtaining the firstmeasurement value and the second measurement value includes obtainingthe first measurement value such that it is proportional to themagnitude V1 of the auxiliary voltage V_(AUX) during the on-time andobtaining the second measurement value such that it is proportional tothe magnitude V2 of the auxiliary voltage V_(AUX) during the off-time.The magnitudes V1, V2 of auxiliary voltage V_(AUX) during the on-timeand the off-time are also illustrated in FIG. 3 . Obtaining the secondmeasurement value during the off-time may include obtaining the secondmeasurement value between the beginning of the off-time and thedemagnetization time instance t_(DEMAG), that is, during a time periodin which the inductor voltage is essentially proportional to the outputvoltage V_(OUT).

In the examples explained below, measuring the inductor voltage includesmeasuring the auxiliary voltage V_(AUX). This, however, is only anexample. The inductor voltage may be measured in any other way as well,such as by measuring the input voltage VIK and the switch voltage V22and calculating a difference.

Referring to the above, the auxiliary voltage V_(AUX) is proportional tothe input voltage V_(IN) during the on-time and proportional to theoutput voltage V_(OUT) during the off-time. It can be assumed that theinput voltage V_(IN) and the output voltage V_(OUT) change slowly andcan be considered to be essentially constant over several drive cyclesof the drive signal. Thus, the first measurement value and the secondmeasurement value may be obtained in the same drive cycle. However, itis also possible to obtain the first measurement value and the secondmeasurement value in different drive cycles. Further, “adjusting aduration of the on-time in at least one subsequent drive cycle” (a) mayinclude using the sum of a first measurement value and a secondmeasurement value to adjust the on-time in only one drive cycle beforeobtaining new first and second measurement values, (b) or may includeusing the sum of a first measurement value and a second measurementvalue to adjust the on-time in several successive drive cycles beforeobtaining new first and second measurement values. It is even possibleto obtain one first measurement value based on measurements in severaldrive cycles and to obtain one second measurement value based onmeasurements in several drive cycles. Obtaining one first or secondmeasurement value based on measurements in several drive cycles mayinclude forming an average of measurement values obtained by themeasurements.

One example of a control circuit 4 configured to operate the powerconverter in a method according to FIG. 5 is illustrated in FIG. 6 .This control circuit 4 includes a drive circuit 5 that is configured togenerate the drive signal S_(DRV), a voltage measurement circuit 6 thatis configured to measure the auxiliary voltage V_(AUX), and a crossingdetection circuit 7 that is configured to detect time instances when theauxiliary voltage V_(AUX) crosses a predefined voltage level.

Referring to FIG. 6 , the crossing detection circuit 7 includes acomparator 71 that is configured to compare the auxiliary voltageV_(AUX) with a reference voltage V_(REF1) provided by a referencevoltage source 72. An output signal S_(ZCD) of the comparator 71 isdependent on whether the auxiliary voltage V_(AUX) is higher or lowerthan the reference voltage V_(REF1). According to one example, thereference voltage is zero. In this example, the reference voltage source72 may be omitted and the comparator output signal S_(ZCD) indicateswhether the auxiliary voltage V_(AUX) is higher or lower than zero.Independent of whether the reference voltage V_(REF1) is zero ordifferent from zero, the comparator output signal S_(ZCD) will bereferred to as zero crossing detection (ZCD) signal in this case.Referring to FIG. 3 , in which the zero crossing detection signalS_(ZCD) is also illustrated, the zero crossing detection signal S_(ZCD)indicates the time instances when the auxiliary voltage V_(AUX) crossesthe reference voltage V_(REF1).

After the demagnetization time instance t_(DEMAG), the primary voltageV21 ₁ increases and the auxiliary voltage V_(AUX) decreases, so that theauxiliary voltage V_(AUX) crosses the reference voltage V_(REF1) for thefirst time, Thus, the zero crossing detection signal S_(ZCD), which isgenerated based on comparing the auxiliary voltage V_(AUX) with thereference voltage V_(REF1), indicates when the transformer 21 has beencompletely demagnetized during the off-time.

The voltage measurement circuit 6 is configured to measure the auxiliaryvoltage V_(AUX) in the on-time in order to obtain the first measurementvalue representing the magnitude V1 and during the off-time in order toobtain the second measurement value representing the magnitude V2.Further, the voltage measurement circuit 6 is configured to output avoltage measurement signal S₁₂ that represents the sum of the twomagnitudes V1, V2, that is, the output signal S₁₂ is proportional toV1+V2. The drive circuit 5 receives the ZCD signal S_(ZCD), the voltagemeasurement signal S₁₂ and the feedback signal S_(FB) and generates thedrive signal S_(DRV) based on these signals S_(ZCD), S₁₂, S_(FB). Oneexample of the drive circuit 5 is illustrated in FIG. 7 .

The drive circuit 5 shown in FIG. 7 includes a latch 51, wherein thelatch 51 provides the drive signal S_(DRV). Optionally, a driver (notillustrated) is connected between the latch 51 and the switch 22 and isconfigured to generate the drive signal S_(DRV) suitable for driving theswitch 22 based on an output signal of the latch 51. The latch 51receives an on-time start signal S_(START) and an on-time end signal S8and is configured to generate the on-level of the drive signal S_(DRV)dependent on the on-time start signal and the on-time end signal S8. Theon-time start signal S_(START) is briefly referred to as start signaland the on-time end signal is briefly referred to as end signal in thefollowing.

Just for the purpose of illustration, the latch 51 is an SR-flip-flop inthe example shown in FIG. 7 and receives the start signal S_(START) at aset input S and the end signal S8 at a reset input R. According to oneexample, the start signal S_(START) is the zero crossing detectionsignal S_(ZCD) and the latch 51 is configured to generate the on-levelof the drive signal S_(DRV) when the zero crossing detection signalS_(ZCD) indicates that the transformer has been completely demagnetized.In the example explained before, a falling edge of the zero crossingdetection signal S_(ZCD) during the off-time indicates that thetransformer 21 has been completely demagnetized, so that latch 51 maystart generating the on-level of the drive signal S_(DRV) when a fallingedge of the start signal S_(START) occurs.

Optionally, the start signal S_(START) is not the zero crossingdetection signal S_(ZCD), but a delayed version of the zero crossingdetection signal S_(ZCD) provided by a delay element 52. In some casesit may be desirable not to switch on when the auxiliary voltage V_(AUX)crosses zero, but shortly after the auxiliary voltage V_(AUX) hascrossed zero. This is illustrated in FIG. 3 . As can be seen from FIG. 3, the switch voltage V22 further decreases after the auxiliary voltageV_(AUX) has crossed zero. By switching on the electronic switch 22 afterthe auxiliary voltage V_(AUX) has crossed zero, switching losses thatmay occur in the switch 22 can be reduced. After the transformer 21 hasbeen completely demagnetized, parasitic oscillations may occur. This isexplained in further detail herein below.

Basically, switching losses in the electronic switch 22 can be reducedwhen detecting the time instance at which the transformer has beencompletely demagnetized and when switching on after a delay time thatessentially equals one half (50%) of the duration of one period of theparasitic oscillations. After this delay time, the voltage V22 acrossthe switch 22 has reached a local minimum, which is often referred to asvalley. In the quasi-resonant mode illustrated in FIG. 3 , the switch 22switches when the switch voltage V22 has reached the first valley afterthe demagnetization time instance t_(DEMAG)—The duration of one periodof the parasitic oscillations, which results from an inductance of thetransformer 21 and parasitic capacitors, such as an output capacitanceof the switch 22, can be obtained by measurements or simulations.Referring to FIG. 3 , the transformer 21 is completely demagnetized whenthe primary voltage V21 ₁ starts to increase and the auxiliary voltageV_(AUX) starts to decrease. A first zero crossing of the auxiliaryvoltage V_(AUX) occurs one quarter (25%) of one period of the parasiticoscillations after the transformer 21 has been completely demagnetized,so that switching on the switch 22 one half period of the oscillationsafter the transformer 21 has been completely demagnetized is equivalentto switching on the electronic switch 22 one quarter of one period ofthe oscillations after the auxiliary voltage V_(AUX) has crossed zero.According to one example, a delay time of the delay element 52 shown inFIG. 7 essentially equals one quarter of a period of the parasiticoscillations in order to achieve switching on when the voltage V22across the switch 22 has reached a local minimum (valley).

Referring to FIG. 7 , the end signal S8 is provided by an on-timecontrol circuit 8, wherein the on-time control circuit 8 receives thestart signal S_(START), the feedback signal S_(FB), and the voltagemeasurement signal S₁₂. The on-time control circuit 8 controls theduration of the on-time and causes the latch 51 to generate theoff-level of the drive signal S_(DRV) after a time period defined by theon-time control circuit 8, wherein this time period is dependent on thevoltage measurement signal S₁₂ and the feedback signal S_(FB). Oneexample of the on-time control circuit 8 is illustrated in FIG. 8 .

The on-time control circuit 8 illustrated in FIG. 8 includes a rampsignal generator 83 that receives the start signal S_(START) and isconfigured to output a ramp signal S_(RAMP), a multiplier 82 thatreceives the feedback signal S_(FB) and the voltage measurement signalS₁₂, and a comparator 81 that receives the ramp signal S_(RAMP) and amultiplier output signal S82, which is also referred to as referencesignal in the following. The end signal S8 is available at an output ofthe comparator 81.

FIG. 9 shows signal diagrams that illustrate the functionality of theon-time control circuit 8 shown in FIG. 8 . Referring to FIG. 9 , theramp signal generator 83 is configured to start a new ramp each time thestart signal S_(START) has a certain edge that causes the latch (51 inFIG. 7 ) to start the on-time. Just for the purpose of illustration,this edge of the start signal S_(START) is a falling edge in the exampleshown in FIG. 9 . A predefined edge of the end signal S8 is generatedwhen a signal level of the ramp signal S_(RAMP) reaches a signal levelof an output signal S82 of the multiplier. The predefined edge of theend signal S8 causes the latch (51 in FIG. 7 ) to end the on-time, thatis, to change the signal level of the drive signal from the on-level tothe off-level. Just for the purpose of illustration, the predefined edgeof the end signal S8 that causes the end of the on-time is a rising edgein the example illustrated in FIG. 9 .

Referring to FIG. 8 , the ramp signal generator 83 may receive the endsignal S8 in order to reset the ramp signal S_(RAMP) to a start level,wherein the start level is the signal level from which the ramp signalS_(RAMP) again starts to increase when the start signal S_(START)indicates the beginning of a new on-time.

In the on-time control circuit 8 illustrated in FIG. 8 , a time periodbetween a time instance at which the predefined edge of the start signalS_(START) is received and the time instance at which the predefined edgeof the end signal S8 is generated is proportional to the multiplieroutput signal S82. The multiplier output signal S82, however, isproportional to the voltage measurement signal S₁₂. Thus, for a givenfeedback signal S_(FB) this time duration increases or decreasesproportionally to the voltage measurement signal S₁₂. This time durationequals the duration TON of the on-time.

FIG. 10 illustrates a modification of the on-time control circuit 8shown in FIG. 8 . The on-time control circuit 8 shown in FIG. 10additionally includes a function generator 84 that receives the feedbacksignal S_(FB) and outputs a signal that is exponentially dependent onthe feedback signal S_(FB). The multiplier 82 receives a functiongenerator output signal S84 in this example. Using a signal that isexponentially dependent on the feedback signal S_(FB) instead of thefeedback signal S_(FB) is basically known and, for example, disclosed inDE 197 25 842 A1. Thus, no further explanation is required in thisregard. The on-time controlled by the variable on-time control circuit 8shown in FIG. 10 is proportional to the voltage measurement signal S₁₂and proportional to e^(S) ^(FB) , wherein e is Euler's number.

Instead of using the signal generator 84 shown in FIG. 10 , whichcalculates e^(S) ^(FB) , two signal generators (function generators) 85,86 may be implemented, as illustrated in FIG. 11 . A first one 85 ofthese signal generators 85, 86 generates a first output signal S_(N)that is dependent on the feedback signal S_(FB) such that the firstoutput signal S_(N) increases as the feedback signal S_(FB) increases.This first function generator output signal S_(N) is received by themultiplier 82. A second one 86 of the function generators is configuredto generate a second output signal SD such that the second output signalSD decreases as the feedback signal S_(FB) increases. This second outputsignal SD is received by the ramp signal generator 83 and is configuredto adjust a slope of the ramp signal S_(RAMP). One example of this rampsignal generator 83 is illustrated in FIG. 12 .

The ramp signal generator 83 according to FIG. 12 includes a currentsource 831 and a capacitor 832 connected in series with the currentsource 831. The current source 831 is controlled by the second functiongenerator signal SD such that a current I831 of the current source 831increases as the function generator signal SD increases and decreases asthe function generator signal SD decreases. According to one example,the current I831 is essentially proportional to the function generatorsignal SD. A voltage V832 across the capacitor 832 forms the ramp signalS_(RAMP) in this ramp signal generator 83. A switch 833 connected inparallel with the capacitor 832 is controlled by a logic 834 dependenton the start signal S_(START) and the end signal S8. According to oneexample, the logic 834 is configured to reset the ramp signal S_(RAMP)by closing the switch 833 each time the end signal S8 indicates that theend of the on-time has been reached. Further, the logic is configured tostart a new ramp of the ramp signal S_(RAMP) by opening the switch 933when the start signal S_(START) indicates that the on-time of a newdrive cycle has started.

The function generators 85, 86 can be digital function generators,wherein the ramp signal generator 83 and the comparator may beimplemented using analog circuits. In this case, digital-to-analogconverters (DACs) may be connected downstream the signal generators 85,86. According to one example, the multiplier 82 may be implemented asmultiplying DAC, so that DAC and multiplier is implemented by the samecircuit.

FIG. 13 illustrates one example of the first signal generator outputsignal S_(N) and the second signal generator output signal SD. Referringto FIG. 13 , each of these signals may be a piecewise linear signal,wherein the first signal S_(N) increases as the feedback signal S_(FB)increases, wherein slopes of the piecewise linear sections (a.k.a.,portions) increase as the feedback signal S_(FB) increases. The secondsignal SD decreases as the feedback signal S_(FB) decreases, wherein aslope of this second signal SD at first increases and then decreases. Byimplementing the on-time control circuit 8 in accordance with theexamples illustrated in FIGS. 11, 12 and 13 , the duration of theon-time is proportional to the voltage measurement signal S₁₂ andapproximately proportional to e^(S) ^(FB) .

Referring to the above, the voltage measurement circuit 6 measures theauxiliary voltage V_(AUX) during the on-time and the off-time in one ormore drive cycles and is configured to output the voltage measurementsignal S₁₂ such that it is proportional to the sum of the magnitudes V1,V2 of the auxiliary voltage V_(AUX) during the on-time and the off-time.

One example of the voltage measurement circuit 6 is illustrated in FIG.14 . This voltage measurement circuit 6 includes a first capacitor 601connected in series with a first switch 603 and a second switch 604, anda second capacitor 602 connected in series with a third switch 605 and afourth switch 606, a fifth switch 607 connected between the first andsecond capacitors 601, 602, a sixth switch 608 connected to the firstcapacitor 602, a sample-and-hold (S/H) circuit 609 coupled to the secondcapacitor 602, and a logic 610 configured to control operation of theswitches 603-608 and the S/H-circuit 609. A first series circuitincluding the first capacitor 601 and the first and second switches 603,604 and a second series circuit including the second capacitor 602 andthe third and fourth switches 605, 606 are each connected between anauxiliary voltage input 42 and a ground input 43 of the control circuit4. At the auxiliary voltage input 42, the control circuit 4 is connectedto the auxiliary winding 23, and the ground input 43 is connected to theground node GND, so that the auxiliary voltage V_(AUX) is availablebetween the auxiliary voltage input 42 and the ground input 43.

The logic 610 is configured to receive the drive signal S_(DRV) and isconfigured to control the first and second switch 603, 604 such that thefirst capacitor 601 is connected between auxiliary voltage input 42 andthe ground input 43 for a first time period during the on-time so thatat the end of this time period a voltage across the first capacitor 601equals the magnitude V1 of the auxiliary voltage V_(AUX) during theon-time. Referring to the above, the auxiliary voltage V_(AUX) isnegative during the on-time, so that, after the first time period, apotential at a first capacitor node, which is the capacitor nodeconnected to the auxiliary voltage input 42 by the first switch 603during the first time period, is negative relative to a potential at asecond capacitor node, which is the capacitor node connected to thesecond electronic switch 604.

Further, the logic 609 is configured to control the third and fourthswitch 605, 606 such that the second capacitor 602 is connected betweenauxiliary voltage input 42 and the ground input 43 for a second timeperiod during the off-time so that at the end of this second time perioda voltage across the second capacitor 602 equals the magnitude V2 of theauxiliary voltage V_(AUX) during the off-time. Referring to the above,the auxiliary voltage V_(AUX) is positive during the off-time, so that,after the second time period, a potential at a first capacitor node,which is the capacitor node connected to the auxiliary voltage input 42by the third switch 605 during the second time period, is positiverelative to a potential at a second capacitor node, which is thecapacitor node connected to the fourth electronic switch 604.

Referring to FIG. 14 , the fifth switch 607 is connected between thesecond capacitor node of the first capacitor 601 and the secondcapacitor node of the second capacitor 602 and the sixth switch isconnected between the first capacitor node of the first capacitor 602and the ground input 43. Further, the S/H circuit 609 is connected tothe first capacitor node of the second capacitor 602. The logic 610 isfurther configured, after the second time period and before the off-timeends, to close the fifth and sixth switch 607, 608 (while the remainderof the switches are open) and to activate the S/H circuit to sample thevoltage at the first capacitor node of the second capacitor 602. Avoltage at the first capacitor node of the second capacitor 602 equalsthe magnitude V1 of the auxiliary voltage V_(AUX) sampled by the firstcapacitor 601 during the first time period plus the magnitude V2 of theauxiliary voltage V_(AUX) sampled by the second capacitor 602 during thesecond time period.

Another example of a voltage measurement circuit 6 is illustrated inFIG. 15 . In this example, the voltage measurement circuit 6 includes avoltage limiter 61 and a resistor 62 connected in series between theauxiliary voltage input 42 and the ground input 43 of the controlcircuit 4. The control circuit 4 may be implemented as an integratedcircuit (IC). In this case, the resistor 62 may be an external resistorconnected between the auxiliary winding 23 and a respective input of theIC.

Referring to FIG. 15 , the voltage measurement circuit 6 furtherincludes a current sensor 63 configured to measure a current I_(AUX)from the auxiliary winding 23 via the resistor 62 and the voltagelimiter 61 to the ground input 43. This current I_(AUX) is also referredto as auxiliary current in the following. An evaluation circuit 64receives a current measurement signal S63 that represents the auxiliarycurrent I_(AUX) from the current sensor and outputs the voltagemeasurement signal S₁₂.

When the voltage measurement circuit 6 is implemented as illustrated inFIG. 15 , the crossing detection circuit 7 may monitor a voltage V_(ZCD)across the voltage limiter 61 instead of the auxiliary voltage. Thevoltage limiter is configured to clamp the voltage V_(ZCD) to a first(positive) level when the auxiliary voltage V_(AUX) is positive and asecond (negative) level when the auxiliary voltage V_(AUX) is negative.According to one example, these voltage levels are higher than thereference voltage V_(REF1). The voltage V_(ZCD) across the voltagelimiter 61, which may be referred to as clamped auxiliary voltage orcrossing detection voltage, is not proportional to the auxiliary voltageV_(AUX). This voltage V_(ZCD), however, is below the reference voltageV_(REF1) each time the auxiliary voltage V_(AUX) is below the referencevoltage V_(REF1). Thus, the voltage V_(ZCD) across the voltage limitercan be used to detect time instances when the auxiliary voltage V_(AUX)crosses the reference voltage V_(REF1). In each case, the crossingdetection signal S_(ZCD) output by the crossing detection circuit 7represents those time instances at which the auxiliary voltage V_(AUX)crosses the first reference voltage V_(REF1), such as zero.

When the auxiliary voltage V_(AUX) is higher than the voltage limitdefined by the voltage limiter 61, an auxiliary current I_(AUX) flowsthrough the voltage limiter 61. In the example shown in FIG. 15 , amagnitude of the auxiliary current I_(AUX) is essentially given by

$\begin{matrix}{{I_{AUX} = {\frac{V62}{R62} = \frac{V_{AUX} - V_{ZCD}}{R62}}},} & (1)\end{matrix}$where R62 is a resistance of the resistor 62 and V_(ZCD) is the voltageacross the voltage limiter 61. According to one example, the voltagelimiter 61 is implemented such that the voltage V_(ZCD) across thevoltage limiter 61 is significantly lower than the auxiliary voltageV_(AUX) so that voltage across the voltage limiter is negligiblecompared to the auxiliary voltage. In this case, the auxiliary currentI_(AUX) measured by the current sensor 63 is essentially proportional tothe auxiliary voltage V_(AUX), so that by measuring the auxiliarycurrent I_(AUX) the auxiliary voltage V_(AUX) and, therefore, thevoltage measurement signal S12 can be obtained. FIG. 16A illustrates oneexample of the voltage limiter 61 and the current sensor 63 and FIG. 16Billustrates one example of the evaluation circuit 64.

Referring to FIG. 16A, the voltage limiter 61 includes a firsttransistor N1 connected between the auxiliary voltage input 42 and theground input 43 of the drive circuit 4. The first transistor N1 clampsthe crossing detection voltage V_(ZCD). Driving the first transistor N1in an on-state so that it clamps the crossing detection voltage V_(ZCD)is explained in detail herein further below.

The voltage limiter 61 further includes a second transistor N2 connectedbetween a first circuit node M1 and the ground node 12, and a thirdtransistor N3 connected between the first circuit node M1 and theauxiliary voltage input 42. Further, a current regulator is configuredto control a current I_(M1) into the first circuit node M1 in twodifferent ways dependent on a polarity of the auxiliary voltage V_(AUX).The current I_(M1) into the first circuit node is provided from a supplynode M3 where a supply voltage VDD is available. The supply voltage maybe provided by power supply circuit (not shown in the drawings) in thecontrol circuit 4. This power supply may generate the supply voltage VDDbased on the auxiliary voltage V_(AUX) or based on any other inputvoltage received by the control circuit 4.

When the auxiliary voltage V_(AUX) is positive, the regulator controlsthe current I_(M1) into the first circuit node M1 such that the firsttransistor N1 and the second transistor N2 are operated in the sameoperating point and a current I_(N3) through the third transistor N3 iszero. In this case, a current level of a current I_(N2) through thesecond transistor N2 is proportional to a current I_(N1) through thefirst transistor N1, wherein the current I_(N1) through the firsttransistor N1 equals the auxiliary current I_(AUX). According to oneexample, the first transistor N1 and the second transistor N2 have thesame length and the same width. In this case, the current I_(N2) throughthe second transistor N2 and the auxiliary current I_(AUX) have the samecurrent level.

According to one example, the regulator includes an operationalamplifier OP1 that has a first input connected to the first circuit nodeM1 and second input connected to a second circuit node M2. A switch S2that is controlled by a comparator K1 connects the second circuit nodeM2 to the second input 42 when the auxiliary voltage V_(AUX) has thefirst polarity. In this case, the operational amplifier OP1 controls thecurrent I_(M1) into the first circuit node M1 such that the potential atthe first circuit node M1 equals the potential at the second input 42,so that a voltage across the third transistor N3 is zero. Thus, asmentioned above, the current I_(N3) through the third transistor N3 iszero.

The regulator further includes a variable resistor N7 driven by theoperational amplifier OP1 and connected between the supply node M3 andthe ground node 12, and a current mirror P1, P2. According to oneexample, the variable resistor N7 includes a transistor driven by theoperational amplifier. Optionally, a filter is connected between anoutput of the operational amplifier OP1 and the transistor of thevariable resistor N7. This filter may include a series circuit with aresistor R and a capacitor C.

The current mirror P1, P2 is configured to mirror a current I_(N7)through the variable resistor N7 such that the current I_(M1) into thefirst circuit node M1 is proportional to the current I_(N7) through thevariable resistor N7, wherein a proportionality factor between thecurrent through the variable resistor N7 and the current I_(M1) into thefirst circuit node M1 is given by a current mirror ratio of the currentmirror. According to one example, the current mirror ratio is 1:1, sothat the current I_(M1) into the first circuit M1 equals the currentI_(N7) through the variable resistor N7. The regulator, via the variableresistor N7 and the current mirror P1, P2, adjusts the current I_(M1)into the first circuit node M1 such that the current I_(N3) through thethird transistor N3 is zero, so that—in a steady state of theregulator—the current I_(M1) into the first circuit node M1 equals thecurrent I_(N2) through the second transistor N2, wherein the latter isproportional to the auxiliary current I_(AUX). Thus, given theproportionality between the current I_(M1) into the first circuit nodeM1 and the current I_(N7) through the variable resistor N7, the currentthrough the variable resistor N7 is proportional to the auxiliarycurrent I_(AUX).

Referring to FIG. 16A, the voltage limiter 61 further includes a furthertransistor N5 connected between an output transistor P2 of the currentmirror P1, P2 and the first circuit node M1. This further transistor N5is connected as a diode. That is, a drain node of the further transistorN5 is connected to its gate node. Further, gate nodes of the first,second and third transistors N1, N2, N3 are connected to the drain/gatenode of the further transistor, so that these transistors N1, N2, N3have the same electrical potential at their respective gate node. Thefirst transistor N1 and the second transistor N2 are activated, so thatthey may conduct a current, as soon as the potential at their respectivegate node reaches a level at which gate-source voltage of thesetransistors N1, N2 becomes higher than the respective thresholdvoltages. According to one example, the first, second and thirdtransistor N1-N3 have the same threshold voltage. The threshold voltageof the fifth transistor N5 may be lower than the threshold voltages ofthe first, second and third transistor N1, N2, N3.

When the auxiliary voltage V_(AUX) is negative the switch S2 controlledby the comparator K1 connects the second circuit node M2 to the groundinput 43 and the third transistor N3 is regulated such that thepotential at the first circuit node M1 equals ground potential GND. Theauxiliary current I_(AUX) is negative, that is, the auxiliary currentI_(AUX) flows in a direction opposite the direction illustrated in FIG.16A. Further, in this operating state, the auxiliary current I_(AUX) isgiven by the current I_(N3) through the third transistor N3 minus thecurrent I_(N1) through the first transistor N1. The current I_(N2)through the second transistor N2 is zero, and the current I_(M1) intothe first circuit node M1 equals the current I_(N3) through the thirdtransistor N3. The current I_(N7) through the variable resistor N7 isagain proportional to the current I_(M1) into the first circuit node. Asthe first transistor N1 and third transistor N3 are operated in the sameoperating point, the current I_(N3) through the third transistor N3 isproportional to the auxiliary current I_(AUX) wherein a proportionalityfactor is dependent on a size of the first transistor N1 and a size ofthe third transistor N3.

Referring to the above, when the auxiliary voltage V_(AUX) is positive,the current I_(N2) through the second transistor N2 is proportional tothe auxiliary current I_(AUX), wherein a proportionality factor isdependent on a size of the first transistor N1 and a size of the secondtransistor N3. Further, when the auxiliary voltage V_(AUX) is negative,the current I_(N3) through the third transistor N3 is proportional tothe auxiliary current I_(AUX), wherein a proportionality factor isdependent on a size of the first transistor N1 and a size of the thirdtransistor N3. The sizes of the first, second and third transistors canbe adapted to one another such that a proportionality factor between amagnitude of the auxiliary current I_(AUX) and the current I_(M1) intothe first circuit node M1 is the same when the auxiliary voltage V_(AUX)(and the auxiliary current I_(AUX)) is positive and when the auxiliaryvoltage V_(AUX) (and the auxiliary current I_(AUX)) is negative. In thiscase, these transistors N1, N2, N3 are implemented with different sizes.

According to another example, the first, second and third transistorsN1, N2, N3 have the same size and the current sensor additionallyincludes a further transistor N6. This further transistor N6 isconnected in parallel with the first transistor N1 only when theauxiliary voltage V_(AUX) is positive. This is achieved by a furtherswitch S3 connected in series with the further transistor N6 andcontrolled by the comparator K1. The further transistor N6 is controlledin the same way as the first transistor N1 by the potential at the thirdcircuit node M3. In this circuit, when the auxiliary voltage V_(AUX) ispositive, the current I_(N1) through the first transistor N1 and acurrent I_(N6) through the further transistor N6 each equals 50% of theauxiliary current I_(AUX). Further, the current I_(N2) through thesecond transistor N2 and the current I_(M1) into the first circuit nodeM1 equal 50% of the auxiliary current I_(AUX). When the auxiliaryvoltage V_(AUX) is negative, the current I_(N3) through the thirdtransistor and, therefore, the current into the first circuit node M1also equals 50% of the auxiliary current, so that the proportionalityfactor between the magnitude of the auxiliary current I_(AUX) and thecurrent I_(M1) into the first circuit node M1 and, therefore, theproportionality factor between the magnitude of the auxiliary currentI_(AUX) and the current I_(N7) through the variable resistor N7 is thesame when the auxiliary voltage V_(AUX) is positive and when theauxiliary voltage V_(AUX) is negative.

The clamping voltage, which is the voltage level at which the crossingdetection voltage V_(ZCD) is clamped by the voltage limiter 61, ispredefined, but not fixed. This clamping voltage is defined by thecharacteristic curve of the first transistor N1 and is dependent on acurrent level of the auxiliary current I_(AUX). Thus, at each voltagelevel of the auxiliary voltage V_(AUX) the clamping voltage ispredefined by the first transistor N1, but not fixed. Basically, thehigher the auxiliary current I_(AUX) the higher the clamping voltage.However, there is a square relationship between the clamping voltage andthe auxiliary current I_(AUX) so that variations of the clamping voltagedependent on the auxiliary current I_(AUX) are essentially negligible.

According to one example, the first transistor N1 is implemented suchthat a voltage drop across the first transistor N1 is less than 5% oreven less than 2% of the auxiliary voltage V_(AUX) during thedemagnetization phase. In this case, despite variations, a magnitude ofthe crossing detection voltage V_(ZCD) is almost negligible compared tothe magnitude of the auxiliary voltage V_(AUX), so that in each case theauxiliary current I_(AUX) is essentially proportional to the auxiliaryvoltage V_(AUX).

In the current sensor 63 illustrated in FIG. 16A, the current I_(N7)through the transistor forming the variable resistor N7 and theelectrical potential at a gate node G of this transistor N7 representthe auxiliary current I_(AUX) and, therefore, the auxiliary voltageV_(AUX). More specifically, the current I_(N7) is proportional to amagnitude of the auxiliary current I_(AUX). That is, current I_(N7),independent of a current flow direction of the auxiliary currentI_(AUX), always flows in the same direction and has a signal level thatis proportional to the magnitude of the auxiliary current I_(AUX). Thepotential at the gate node forms the current measurement signal S63 inthis example. An evaluation circuit 64 configured to generate themeasurement signal S₁₂ based on this current measurement signal S63 isillustrated in FIG. 16B. The gate node G of the transistor N7 is alsoreferred to as output of the current sensor in the following, and thetransistor N7 is also referred to as output transistor of the currentsensor.

The evaluation circuit shown in FIG. 16B includes a first capacitor C1coupled to the output G of the current sensor 63 through a first switchSHP and a second capacitor C2 coupled to the output G of the currentsensor 63 through a second switch SHS. The first switch SHP and thesecond switch SHS are controlled by a logic 641 dependent on the drivesignal S_(DRV). The logic 641 is configured to control the first switchSHP such that the first capacitor C1 is connected to the output G of thecurrent sensor 63 for a first time period during the on-time so that atthe end of this first time period a voltage VC1 across the firstcapacitor C1 equals the voltage at the output G of the current sensor63. The logic 641 is further configured to control the second switch SHSsuch that the second capacitor C2 is connected to the output G of thecurrent sensor 63 for a second time period during the off-time so thatat the end of this second time period a voltage VC2 across the secondcapacitor C2 equals the voltage at the output G of the current sensor63.

Referring to FIG. 16B, the evaluation circuit 64 further includes afirst transistor N8 driven by the voltage VC1 across the first capacitorC1 and a second transistor N9 driven by the voltage VC2 across thesecond capacitor C2. According to one example, the first transistor N8and the second transistor N9 have the same size as the output transistorN7 of the current sensor 63. In this case, a first current INS throughthe first transistor N8 equals the current I_(N7) through the outputtransistor N7 of the current sensor 63 during the first time period,wherein this current is proportional to the auxiliary voltage V_(AUX)during the on-time and, therefore, proportional to the first magnitudeV1 explained above. Further, a second current I_(N9) through the secondtransistor N9 equals the current I_(N7) through the output transistor N7of the current sensor 63 during the second time period, wherein thiscurrent is proportional to the auxiliary voltage V_(AUX) during theoff-time and, therefore, proportional to the second voltage V2 explainedabove.

Referring to FIG. 16B, the evaluation circuit 64 further includes afirst current mirror 642 having an input connected to the firsttransistor N8 and a second current mirror 643 having an input connectedto the second transistor N9. Outputs of the current mirrors 642, 643 areconnected. Further, the current mirrors 642, 643 may have the samecurrent mirror ratio so that an output current I64 of the currentmirrors 642, 643 is proportional to a sum of the first and secondcurrents INS, I_(N9) and, therefore, proportional to the sum of thefirst magnitude V1 and the second magnitude V2. This output current I64may form the voltage measurement signal S12. Alternatively, asillustrated in dashed lines in FIG. 16B, a resistor 65 is connected inseries with the output of the current mirrors 642, 643. In this case,the voltage measurement signal S12 is a voltage across the resistor 65.

FIG. 3 illustrates operating the power converter in the quasi-resonantmode. In this operating mode, a delay time T_(DEL) between thedemagnetization time instance t_(DEMAG) and the end of the off-time isabout one half of one oscillation period T_(OSC). Further, in thismethod, the on-time is adjusted dependent on the first measurementsignal S₁₂. In this method, the shorter the delay time T_(DEL) relativeto overall duration T of the drive cycle, the more accurate the waveformof the input current I_(IN) can be controlled.

Another type of operating mode, which is referred to as second operatingmode in the following, is explained in the following. In this operatingmode, the delay time T_(DEL) is considered when adjusting the durationTON of the on-time. In this way, the waveform of the input currentI_(IN) can be regulated in an accurate way, even when the delay timeT_(DEL) is not negligible as compared to the overall time duration T ofthe drive cycle. The second operating mode may include operating thepower converter in the quasi-resonant (QR) mode or in a valley skippingQR mode. In the valley skipping QR mode, one or more valleys of theswitch voltage V22 are skipped before the switch 22 again switches on.In this operating mode, a delay time T_(DEL) between the demagnetizationtime instance t_(DEMAG) and the time instance when the electronic switch22 again switches on is longer than one half of a period of theparasitic oscillations. Signal diagrams that are based on the signaldiagrams shown in FIG. 3 and that illustrate operating the powerconverter in the valley skipping quasi-resonant mode are illustrated inFIG. 17 .

As can be seen from FIG. 17 , in the valley skipping quasi-resonantmode, the delay time T_(DEL) may include a significant portion of theoverall time duration T of the drive cycle. In the QR mode and thevalley skipping QR mode, the delay time T_(DEL) between thedemagnetization time instance t_(DEMAG) and the start of a new drivecycle is given by T_(DEL)=(i−0.5)·T_(OSC), wherein T_(OSC) is theduration of one period of the parasitic oscillations and i is the ordernumber of the valley in which the electronic switch 22 switches on,wherein i is an integer and wherein i≥1. Equivalently, i−1 is the numberof valleys that are skipped. The power converter operates in the QR modewhen the electronic switch 22 switches on in the first (i=1) valley andoperates in the valley skipping QR mode when i>1, that is, when one ormore valleys are skipped.

FIG. 18 shows a flowchart of one example of a method for operating thepower converter in the quasi-resonant mode such that the outputparameter has a predefined value and such the waveform of the averageinput current I_(IN_AVG) essentially equals the waveform of the inputvoltage V_(IN). This method is based on the method illustrated in FIG. 5and is different from the method illustrated in FIG. 5 in that theon-time in each drive cycle includes two on-time sections, a firston-time section and a second on-time section. The first on-time section,similar to the on-time in the first operating mode, is dependent on thesum of the first measurement value and the second measurement value andthe feedback signal (104), wherein the first and second measurementvalues are obtained in one or more previous drive cycles. The secondon-time section is dependent on the second measurement value obtained inthe previous drive cycle, the duration of the first on-time section inthe instantaneous drive cycle, and the delay time between thedemagnetization of the transformer and the begin of a new drive cycle inthe previous drive cycle (105).

FIG. 19 illustrates one example of a control circuit 4 configured tooperate the power converter in accordance with the method illustrated inFIG. 18 . This control circuit 4 is based on the control circuit shownin FIG. 6 and is different from the control circuit shown in FIG. 6 inthat the drive circuit 5 additionally receives a further voltagemeasurement signal S₂, wherein this further voltage measurement signalS₂ represents the second measurement value V2, that is, the magnitude ofthe auxiliary voltage V_(AUX) during the off-time. The voltagemeasurement signal S₁₂ is also referred to as first voltage measurementsignal in the following, and the further voltage measurement signal S₂is also referred to as second voltage measurement signal in thefollowing.

One example of the voltage measurement circuit 6 configured to outputthe first voltage measurement signal S₁₂ and the second voltagemeasurement signal S₂ is illustrated in FIG. 20 . This voltagemeasurement circuit 6 is based on the voltage measurement circuitillustrated in FIG. 14 and additionally includes a further S/H circuit611. This further S/H circuit 611 is controlled by the logic 610 andconfigured to sample the voltage V2 across the first capacitor 601 atthe end of the second time period.

According to another example, the voltage measurement circuit 6 is basedon the voltage measurement circuit illustrated in FIGS. 16A and 16B,wherein just one modification in the evaluation circuit 64 is requiredin order to output both the first voltage measurement signal S₁₂ and thesecond voltage measurement signal S₂. One example of a correspondingevaluation circuit 64 is illustrated in FIG. 21 . In this evaluationcircuit 64, the second current mirror 643 includes two outputs, a firstoutput connected to the output of the first current mirror 642, and asecond output providing an output current proportional to the secondcurrent I_(N9) explained above. This output current is proportional tothe second magnitude V2 of the auxiliary voltage V_(AUX) and forms thesecond voltage measurement signal S₂. Alternatively, a resistor 66 isconnected in series to the second current mirror output, so that thesecond voltage measurement signal S₂ is a voltage.

FIG. 22 illustrates one example of the drive circuit 5 shown in FIG. 19. This drive circuit 5 is based on the drive circuit shown in FIG. 7 andadditionally includes a zero crossing controller 53 that receives thefeedback signal S_(FB) and outputs a zero crossing reference signalS_(ZC_REF), a counter 54 that receives the zero crossing referencesignal S_(ZC_REF) and the zero crossing detection signal S_(ZCD), and afurther on-time control circuit 9 connected between the on-time controlcircuit 8 and the latch 51. The on-time control circuit 8 is alsoreferred to as first on-time control circuit and the further on-timecontrol circuit 9 is also referred to as second on-time control circuitin the following. The second on-time control circuit 9 receives the endsignal S8 from the first on-time control circuit 8 and outputs an endsignal S9 to the latch 51. The end signal S8 from the first on-timecontrol circuit 8, which is also referred to as first end signal in thefollowing, indicates an end of the first on-time section, and the endsignal S9 from the second on-time control circuit 9, which is alsoreferred to as second end signal in the following, indicates an end ofthe second on-time section explained above.

The second on-time control circuit 9 further receives the feedbacksignal S_(FB) and the further voltage measurement signal S₂, the startsignal S_(START) and the zero crossing reference signal S_(ZC_REF). Thezero crossing reference signal S_(ZC_REF) represents the delay timeT_(DEL) between the demagnetization time instance t_(DEMAG) and thestart of a new drive cycle, that is, the zero crossing reference signalS_(ZC_REF) represents the number of zero crossings of the auxiliaryvoltage V_(AUX) that are allowed to occur before a new drive cyclestarts. This zero crossing reference signal S_(ZC_REF) is dependent onthe feedback signal S_(FB).

According to one example, the feedback signal S_(FB) is generated suchthat the feedback signal S_(FB) decreases as the power consumption ofthe load decreases. Further, the zero crossing reference signalS_(ZC_REF) may be is generated such that the number of zero crossingsthat are allowed to occur before a new drive cycle start decrease as thefeedback signal S_(FB) increases. One example of such dependency of thezero crossing reference signal S_(ZC_REF) on the feedback signal S_(FB)is illustrated in FIG. 23 .

Referring to FIG. 22 , the counter 54 receives the zero crossingdetection signal S_(ZCD) and the zero crossing reference signalS_(ZC_REF) and is configured to generate the start signal S_(START)when, during the off-time, the number of zero crossings defined by thezero crossing reference signal S_(ZC_REF) has occurred. The delayelement 52 may delay generating the start signal S_(START) for onequarter of one period T_(OSC) of the parasitic oscillations, as alreadyexplained with reference to FIG. 7 .

The first on-time control circuit 8 may be configured in accordance withany of the examples explained herein before. In particular, the firston-time control circuit 8 may be configured in accordance with theexample illustrated in FIG. 10 . One example of the second on-timecontrol circuit 9 is illustrated in FIG. 24 .

The second on-time control circuit 9 shown in FIG. 24 is similar to thefirst on-time control circuit 8 shown in FIG. 10 and includes acomparator 91 that receives a second ramp signal S_(RAMP2) from a rampsignal generator 93 and a reference signal S98, wherein this referencesignal S98 is proportional to the delay time T_(DEL), the secondmeasurement signal S₂, and inversely proportional to the time durationsince the beginning of the instantaneous on-time. This reference signalS98 is provided by a function generator 98 that receives the startsignal S_(START), which includes the information on the beginning of theinstantaneous on-time. Further, the function generator 98 receives asignal that is proportional to the second measurement signal S₂, and thedelay time T_(DEL). The information on the delay time T_(DEL) isincluded in a signal S94 that is obtained from the zero crossingreference signal S_(ZC_REF) by subtracting 0.5 by a subtractor 94. Thissignal S94 is multiplied with the second measurement signal S₂ andeither the feedback signal S_(FB) or a signal dependent on the feedbacksignal.

As explained above, instead of the feedback signal S_(FB) an exponentialfeedback signal e^(S) ^(FB) or, as explained with reference to FIGS. 11and 13 instead of a function generator calculating e^(S) ^(FB) twofunction generators, each implementing a piecewise linear function maybe used. In the example illustrated in FIG. 24 , two such functiongenerators are used. A first function generator 95 and a second functiongenerator 96. These function generators 95, 96 can be the same functiongenerators as the function generators 85, 86 in the first on-timecontrol circuit 8. That is, one first function generator may be used asthe first function generator 85 in the first on-time control circuit 8and the first function generator 95 in the second on-time controlcircuit 9. Equivalently, one and the same function generator may be usedas the second function generator 86 in the first on-time control circuit8 and the second function generator 96 in the second on-time controlcircuit 9. These function generators can be digital function generatorswherein the ramp signal generators 83, 93 may be implemented usinganalog circuit elements. Further, the function generator 98 illustratedin FIG. 24 may be implemented using analog circuit elements. In thiscase, digital-to-analog converters (DACs) may be connected between thefunction generators 96 and 95 and the ramp signal generator 93 and themultiplier 97. When the function generators 85, 95 in the first andsecond on-time control circuits 8, 9 are implemented using only onefunction generator only one DAC is required. Equivalently, when thefunction generators 86, 96 in the first and second on-time controlcircuits 8, 9 are implemented using only one function generator only oneDAC is required.

Referring to FIG. 24 , the multiplier 97 multiplies the first functiongenerator output signal S_(N) with the delay time signal S94. A furthermultiplier 92 multiplies the result of the first multiplier 97 with thefurther measurement value S₂, wherein an output signal S92 of the secondmultiplier 92 is received by the function generator 98. Although FIG. 24shows two multipliers 92, 97, this is only an example. According toanother example, the multiplier 92 is a multiplying digital-to-analogconverter and the multiplier 97 is a digital multiplier.

One example of the ramp signal generator 93 shown in FIG. 24 isillustrated in FIG. 25 . This function generator 93 is implemented inthe same way as the ramp signal generator 83 illustrated in FIG. 12 andincludes a series circuit with a current source 931 and a capacitor 932.The second ramp signal S_(RAMP2) is the voltage V932 across thecapacitor 932. A switch 933 is connected in parallel with the capacitor932 and is controlled by a logic 934 dependent on the first end signalS8 and the second end signal S9. According to one example, the logic 934is configured to close the switch 933 before the first end signal S8indicates that the end of the first on-time section has been reached.When the first end signal S8 indicates that the end of the first on-timesection has been reached, the logic 934 opens the switch 933 in order togenerate a ramp of the ramp signal. The ramp signal generator is resetby the second end signal S9, that is, the switch 933 is closed, when thesecond end signal S9 indicates that the second ramp signal S_(RAMP2) hasreached the second reference signal S98, that is, when the second endsignal S9 indicates that the end of the second on-time section has beenreached.

The function of the drive circuit 5 with the first and second on-timecontrol circuits 8, 9 is illustrated in FIG. 26 . FIG. 26 illustratessignals diagrams of the auxiliary voltage V_(AUX), the reference signalsS82, S98 (which are referred to as first and second reference signals inthe following) and the first and second ramp signals S_(RAMP1),S_(RAMP2). In particular, FIG. 26 illustrates generating the first andsecond on-time sections T_(ON1), T_(ON2) of the on-time. As illustratedin FIG. 26 , the first reference signal S82 in one drive cycle isdependent on the first measurement signal S₁₂ obtained in a previousdrive cycle. The “previous drive cycle” can be the drive cycle directlypreceding the instantaneous drive cycle. This, however, is only anexample. It is also possible, to obtain the first measurement signal S₁₂in one drive cycle and to use this first measurement value S₁₂ in two ormore subsequent drive cycles. Further, the first measurement signal S₁₂the first measurement signal S₁₂ may be generated based on a firstmeasurement value V1 and a second measurement value V2 obtained indifferent preceding drive cycles.

As can be seen from FIG. 26 and as explained before, the first on-timesection T_(ON1) ends when the first ramp signal S_(RAMP1), which startsat the beginning of the instantaneous drive cycle, reaches the firstreference signal S82, wherein the first reference signal S82 isproportional to the first measurement signal S₁₂ and dependent on thefeedback signal S_(FB). The second on-time section T_(ON2) starts whenthe first on-time section T_(ON1) ends, wherein at the beginning of thesecond on-time section T_(ON2) the second ramp signal S_(RAMP2) starts.The second reference signal S98 starts at the beginning of theinstantaneous drive cycle, wherein, as explained above, a start level isgiven by the multiplier output signal S92, which is proportional to thesecond measurement signal S2 and the delay time signal S92 and isdependent on the feedback signal S_(FB). Further, the second referencesignal S98 decreases inversely proportional to the time since thebeginning of the instantaneous drive cycle. The second on-time sectionT_(ON2) ends when the second ramp signal S98, which may increaselinearly, reaches the second reference signal S98. In this case, aduration of the second on-time section T_(ON2) is proportional to thesecond measurement signal S2 and the delay time signal S94. Further, theduration of the second on-time section is inversely proportional to thetime period since the beginning of the on-time. In this way, theduration of the second on-time section T_(ON2) is dependent on theduration of the first on-time section T_(ON1). Thus, for a given secondmeasurement signal S2 and a given delay time signal S94, the longer thefirst on-time section T_(ON1) the shorter the second on time sectionT_(ON2).

Referring to the above, the zero crossing reference signal S_(ZC_REF)changes in steps of one (1) dependent on the feedback signal S_(FB).Consequently, the delay time signal S94 changes in steps of onedependent on the feedback signal. Such change in the zero crossingreference signal S_(ZC_REF), however, does not result in an abruptchange of the output power, because such change is considered in thesecond on-time section T_(ON2). Thus, when the zero crossing referencesignal S_(ZC_REF) increases/decreases the duration of the second on-timesection T_(ON2) increases/decreases.

According to one example, generating the zero crossing reference signalS_(ZC_REF) is synchronized such that the zero crossing reference signaldoes not change during the delay time T_(DEL). For this synchronization,the zero crossing controller 53 illustrated in FIG. 22 may receive thedrive signal S_(DRV). According to one example, the zero crossingreference signal S_(ZC_REF) is updated based on the feedback signalS_(FB) once in each drive cycle. The zero crossing reference signalS_(ZC_REF) may be updated, for example, at the beginning of theoff-time. Basically, it can be assumed that the zero crossing referencesignal S_(ZC_REF) is constant over several drive cycles. Thus, it ispossible to adjust the second on-time section in the instantaneous drivecycle based on the zero crossing reference signal S_(ZC_REF) used in theprevious drive cycle to adjust the delay time or based on the zerocrossing reference signal S_(ZC_REF) that will be used in theinstantaneous drive cycle to adjust the delay time.

On example of the function generator 98 and its functionality areillustrated in FIGS. 27 and 28 . This function generator 98 does notexactly output a signal that is inversely proportional to the time sincethe beginning of the instantaneous drive cycle, but approximates a 1/tfunction using exponential functions. Referring to FIG. 27 , thefunction generator includes a capacitor 981. This capacitor 981 ischarged via an input switch 982 by the signal S92 which is proportionalto the delay time and the second measurement signal S₂ and which isdependent on the feedback signal S_(FB). This input switch 982 is openedby a first delay element 983 at a first time instance t1. After thefirst time instance t1, the capacitor 981 is discharged through aresistor network. The capacitor 981 and the resistor network form an RCelement, wherein an RC constant of the RC element is increased stepwiseover the time so that a discharge rate of the capacitor decreases overthe time and the voltage V981 across the capacitor 981 approximates the1/t function. More specifically, the voltage across the capacitor 981 isapproximately proportional to 1/(t−t0), wherein t0 denotes the timeinstance when the on-time starts. The voltage V981 across the capacitor981 forms the second reference signal S98 in this example.

In the example illustrated in FIG. 27 , the resistor network includesthree resistors, a first resistor 986, a second resistor 987, and athird resistor 988 that are connected in series, wherein the seriescircuit is connected in parallel with the capacitor 981. A first switch989 is connected in parallel with a series circuit formed by the secondand third resistors 987, 988, and second switch 990 is connected inparallel with the third resistor 988.

Before the first time instance t1, the capacitor voltage V981 ismaintained at the voltage level defined by the multiplier output signalS92. Each of the input switch 982 and the first and second switches 989,990 is switched on before the first time instance t1. After the firsttime instance t1, the capacitor 981 is discharged via the first resistor986 and the first switch 989, wherein the first switch 989 shorts thesecond and third resistors 987, 988. At a second time instance t2 thefirst switch 989 is opened by a second delay element 984, and thecapacitor 981 is discharged via the first resistor 986 and the secondresistor 987, wherein the second switch 990 shorts the third resistor988. Finally, at a third time instance t3 the second switch 990 isopened by a third delay element 985, and the capacitor 981 is dischargedvia the first resistor 986, the second resistor 987, and the thirdresistor.

In the function generator illustrated in FIG. 28 , an 1/t function isapproximated using three exponential capacitor discharge functions withdifferent RC time constants such that at least between the first timeinstance t1 and a fourth time instance the capacitor voltage V981 isapproximately proportional to 1/(t−t0). Between the beginning of theon-time at time instance t0 and the first time instance, the capacitorvoltage V981 is constant. According to one example, a time periodbetween the beginning of the on-time and the first time instance t1 isshorter than an expected minimum of the duration of the first on-timesection.

Referring to the above, the power converter, in the first and secondoperating mode regulates the output parameter such that it has apredefined level and regulates the input current I_(IN) such that anaverage waveform of the input current I_(IN) is proportional to theinput voltage V_(IN). This is explained with reference to the secondoperating mode in the following.

In the steady state, the integral of the voltage V21 ₁ across theprimary winding 21 ₁ over one drive cycle is zero,∫₀ ^(T) V 21 ₁ dt=0  (2a)Based on equation (2a), given that the auxiliary voltage V_(AUX) isproportional to the inductor voltage V21 ₁, and considering thewaveforms illustrated in FIG. 17 the following relationship applies forthe first and second measurement values V1, V2, the on-time duration TONand the demagnetization duration T_(DEM):

$\begin{matrix}{{V{1 \cdot T_{ON}}} = {{V{2 \cdot T_{DEM}}} = {{> T_{DEM}} = {T_{ON} \cdot {\frac{V1}{V2}.}}}}} & \left( {3a} \right)\end{matrix}$Further, a peak I2 _(PK) of the inductor current I2 is given by

$\begin{matrix}{{{I2_{PK}} = {\frac{V_{IN}}{L} \cdot T_{ON}}},} & \left( {4a} \right)\end{matrix}$wherein V_(IN) is the input voltage and L is the inductance of theinductor. More specifically, L is the inductance of the primary winding21 ₁ of the transformer 21. Referring to the above, the inductor currentI2 equals the input current I_(IN). Further, referring to FIG. 17 , theinductor current I2 has a triangular waveform during the on-time. Anaverage I_(IN_AVG) of the input current I_(IN) is then given by

$\begin{matrix}{I_{{IN}_{AVG}} = {{\frac{I2_{PK}}{2} \cdot \frac{T_{ON}}{T}} = {\frac{V_{IN}}{2 \cdot L} \cdot T_{ON} \cdot {\frac{T_{ON}}{T}.}}}} & \left( {5a} \right)\end{matrix}$Further, in order to achieve a proportionality between the input voltageV_(IN) and the average input current I_(IN_AVG), an input impedance ofthe power converter should be essentially constant at a given powerconsumption of the load, that is,

$\begin{matrix}{{Z_{IN} = {\frac{V_{IN}}{I_{{IN}_{AVG}}} = c}},} & \left( {6a} \right)\end{matrix}$wherein ZEN denotes the input impedance of the power converter and c isa constant that is dependent on power consumption of the load.Basically, the higher the power consumption of the load, the lower theinput impedance ZEN. Based on equations (3a), (5a) and (6a), the inputimpedance can be expressed as

$\begin{matrix}\begin{matrix}{Z_{IN} = \frac{V_{IN}}{\frac{V_{IN}}{2 \cdot L} \cdot T_{ON} \cdot \frac{T_{ON}}{T}}} \\{= {\frac{2 \cdot L}{T_{ON}} \cdot \frac{T}{T_{ON}}}} \\{= {\frac{2 \cdot L}{T_{ON}} \cdot \frac{T_{ON} + T_{DEM} + T_{DEL}}{T_{ON}}}} \\{= {\frac{2 \cdot L}{T_{ON}} \cdot \frac{T_{ON} + {T_{ON} \cdot \frac{V1}{V2}} + T_{DEL}}{T_{ON}}}} \\{= {\frac{2 \cdot L}{T_{ON}} \cdot \left( {1 + \frac{V1}{V2} + \frac{T_{DEL}}{T_{ON}}} \right)}} \\{= {{\frac{2 \cdot L}{T_{ON}} \cdot \frac{1}{V2}}{\left( {{V2} + {V1} + {V{2 \cdot \frac{T_{DEL}}{T_{ON}}}}} \right).}}}\end{matrix} & \left( {7a} \right)\end{matrix}$Based on equation (7a), the desired duration TON of the on-time can beexpressed as

$\begin{matrix}{T_{ON} = {\frac{2 \cdot L}{{Z_{IN} \cdot V}2} \cdot {\left( {{V2} + {V1} + {V{2 \cdot \frac{T_{DEL}}{T_{ON}}}}} \right).}}} & \left( {8a} \right)\end{matrix}$

In the second operating mode explained above, adjusting the duration TONof the on-time in accordance with equation (8a) is achieved by havingthe first on-time section and the second on-time section. Referring tothe above, the duration T_(ON1) of the first on-time section isproportional to the sum V1+V2 of the first and second measurement valuesV1, V2. In the first on-time controllers 8 illustrated in FIGS. 8, 10and 11 the first measurement signal S₁₂ that is used to adjust theduration T_(ON1) of the first on-time section represents the sum V1+V2of the first and second measurement values V1, V2. In equation (8a), theterm V1+V2 represents the first on-time section.

Referring to the above, the duration T_(ON2) of the second on-timesection is proportional to the second measurement value V2 and the delaytime T_(DEL) and inversely proportional to the duration since thebeginning of the on-time, that is inversely proportional to the durationTON of the on-time. In the second on-time controller 9 illustrated inFIG. 24 , the second measurement signal S₂ represents the secondmeasurement value V2 and the delay time signal S94 represents the delaytime T_(DEL). Further, the signal value of the function generator signalS98 is proportional to the second measurement signal S₂ and the delaytime signal S94, so that duration T_(ON2) of the second on-time sectionis proportional to the delay time T_(DEL) and the second measurementvalue V2. Further, the function generator output signal S98 is inverselyproportional to the time that has elapsed since the beginning of theon-time, so that the duration of the second on-time section isproportional to the duration of the (instantaneous) on-time. In equation(8a), the term

$V{2 \cdot \frac{T_{DEL}}{T_{ON}}}$represents the second on-time section.

Referring to the above, each of the first on-time section and the secondon-time section is dependent on the feedback signal S_(FB) in the sameway. This feedback signal S_(FB) is represented by the term

$\frac{2 \cdot L}{{Z_{IN} \cdot V}2}$in equation (8a).

Referring to equation (8a), when T_(DEL) is short relative to theduration of the on-time, the on-time is approximately given by

$\begin{matrix}{{T_{ON} = {\frac{2 \cdot L}{{Z_{IN} \cdot V}2} \cdot \left( {{V2} + {V1}} \right)}},} & \left( {9a} \right)\end{matrix}$which represents operating the power converter in the first operatingmode.

Although operating a power converter in a first operating mode and asecond operating mode has been explained with reference to a flybackconverter, this is only an example. These operating methods are notrestricted to be used in a flyback converter, but may be used in othertypes of power converters, such as a boost converter as well. An exampleof a boost converter is illustrated in FIG. 29 .

While the inductor 21 in the flyback converter is a transformer, theinductor 21 in the boost converter is a choke, for example, and isconnected in series with the switch 22, wherein the series circuitincluding the inductor 21 and the electronic switch 22 is connected tothe input 11, 12. The rectifier circuit 3 is connected between a circuitnode at which the inductor 21 and the switch 22 are connected and theoutput 13, 14. In this type of power converter, the input voltage V_(IN)and the output voltage V_(OUT) may be referenced to the same potential.Thus, the coupler 16 may be omitted.

The auxiliary winding 23 is inductively coupled to the inductor 21, andthe auxiliary voltage V_(AUX) is proportional to a voltage V21 acrossthe inductor 21.

Like operating the electronic switch in the flyback converter, operatingthe electronic switch 22 in a switched-mode fashion includes operatingthe electronic switch 22 in a plurality of successive drive cycles,wherein in each of these drive cycles the electronic switch 22 switcheson for an on-time and switches off for an off-time. Signal diagrams thatcorrespond to the signal diagrams shown in FIG. 17 , but apply to aboost converter are illustrated in FIG. 30 .

As can be seen from FIG. 30 , operating the boost converter insuccessive drive cycles is very similar to operating the flybackconverter in successive drive cycles. During the on-time, the inductorcurrent I2 decreases and the inductor 21 is magnetized. During theoff-time, the inductor 21 is demagnetized, and parasitic oscillationsoccur during the delay time between the end of the demagnetizationperiod and the start of a new drive cycle. Differences between operatingthe boost converter and operating the flyback converter are: (a) in theboost converter, the inductor current decreases over the demagnetizationperiod T_(DEM); (b) the inductor voltage V21, during the demagnetizationperiod T_(DEM), is essentially given by the input voltage V_(IN) minusthe output voltage V_(OUT); and the switch voltage V22 during thedemagnetization period T_(DEM) essentially equals the output voltageV_(OUT). Nevertheless, the boost converter can be operated in the firstand second operating mode in the same way as the flyback converterexplained above. This can be seen from equations (2b)-(8b) below. Theseequations correspond to equations (2a)-(8a), but apply to the boostconverter.

In the steady state, the integral of the voltage V21 across the inductorover one drive cycle is zero,∫₀ ^(T) V 21 ₁ dt=0  (2b)Based on equation (2b), given that the auxiliary voltage V_(AUX) isproportional to the inductor voltage V21, and considering the waveformsillustrated in FIG. 30 the following relationship applies for the firstand second measurement values V1, V2, the on-time duration T_(ON) andthe demagnetization duration T_(DEM):

$\begin{matrix}{{V{1 \cdot T_{ON}}} = {{V{2 \cdot T_{DEM}}} = {{> T_{DEM}} = {T_{ON} \cdot {\frac{V1}{V2}.}}}}} & \left( {3b} \right)\end{matrix}$Further, a peak I2 _(PK) of the inductor current I2 is given by

$\begin{matrix}{{{I2_{PK}} = {\frac{V_{IN}}{L} \cdot T_{ON}}},} & \left( {4b} \right)\end{matrix}$wherein V_(IN) is the input voltage and L is the inductance of theinductor 21. The inductor current I2 equals the input current I_(IN).Further, referring to FIG. 30 , the inductor current I2 has a triangularwaveform during the on-time T_(ON) and the demagnetization time T_(DEM).An average I_(IN_AVG) of the input current I_(IN) is then given by

$\begin{matrix}{I_{{IN}_{AVG}} = {{\frac{I2_{PK}}{2} \cdot \frac{T_{ON} + T_{DEM}}{T}} = {\frac{V_{IN}}{2 \cdot L} \cdot T_{ON} \cdot {\frac{T_{ON} + T_{DEM}}{T}.}}}} & \left( {5b} \right)\end{matrix}$Further, in order to achieve a proportionality between the input voltageV_(IN) and the average input current I_(IN_AVG), an input impedance ofthe power converter should be essentially constant at a given powerconsumption of the load, that is,

$\begin{matrix}{{Z_{IN} = {\frac{V_{IN}}{J_{{IN}_{AVG}}} = c}},} & \left( {6b} \right)\end{matrix}$wherein ZIN denotes the input impedance of the power converter and c isconstant that is dependent on power consumption of the load. Basically,the higher the power consumption of the load, the lower the inputimpedance Z_(IN). Based on equations (3b), (5b) and (6b), the inputimpedance can be expressed as

$\begin{matrix}{\begin{matrix}{Z_{IN} = \frac{V_{IN}}{\frac{V_{IN}}{2 \cdot L} \cdot T_{ON} \cdot \frac{T_{ON} + T_{DEM}}{T}}} \\{= {\frac{2 \cdot L}{T_{ON}} \cdot \frac{T}{T_{ON} + T_{DEM}}}} \\{= {\frac{2 \cdot L}{T_{ON}} \cdot \frac{T_{ON} + T_{DEM} + T_{DEL}}{T_{ON} + T_{DEM}}}} \\{= {\frac{2 \cdot L}{T_{ON}} \cdot \frac{{T_{ON} \cdot \left( {1 + \frac{V1}{V2}} \right)} + T_{DEL}}{T_{ON} \cdot \left( {1 + \frac{V1}{V2}} \right)}}}\end{matrix}{\frac{2 \cdot L}{T_{ON}} \cdot {\frac{{T_{ON} \cdot \left( {{V2} + {V1}} \right)} + {{T_{DEL} \cdot V}2}}{T_{ON} \cdot \left( {{V2} + {V1}} \right)}.}}} & \left( {7b} \right)\end{matrix}$Based on equation (7b), the desired duration T_(ON) of the on-time canbe expressed as

$\begin{matrix}{T_{ON} = {{\frac{2 \cdot L}{Z_{IN}} \cdot \frac{1}{{V1} + {V2}}}{\left( {{V2} + {V1} + {V{2 \cdot \frac{T_{DEL}}{T_{ON}}}}} \right).}}} & \left( {8b} \right)\end{matrix}$

The term in brackets is the same as in equation (8a) so that the firstand second on-time sections in the boost converter can be adjusted inthe same way as in the flyback converter. Just the term

$\frac{2 \cdot L}{Z_{IN}} \cdot \frac{1}{{V1} + {V2}}$in equation (8b), that affects both the first and the second on-timesection is different from the corresponding term

$\frac{2 \cdot L}{Z_{IN}}.\frac{1}{V2}$in equation (8a) in that there is an inverse proportionality to V1+V2instead of only V2. This is due to the different topologies of theflyback converter and the boost converter. The term

$\frac{2 \cdot L}{Z_{IN}} \cdot \frac{1}{{V1} + {V2}}$in equation (8b) represents the feedback signal S_(FB).

The invention claimed is:
 1. A method comprising: driving a switch, theswitch coupled to an inductor in a power converter, the switch beingdriven during successive drive cycles, each of which includes an ON-timeand an OFF-time of controlling the switch; wherein driving the switchcomprises: controlling a magnitude of an ON-time duration of the switchin a second drive cycle depending on a feedback signal, the second drivecycle occurring subsequent to a first drive cycle of the drive cycles;wherein controlling the ON-time duration in the second drive cyclecomprises: adjusting a first on-time portion, TON1, of the ON-timeduration in the second drive cycle based on the feedback signal;obtaining a duration of a delay time between a demagnetization timeinstance in the first drive cycle and an end of the first drive cycle;and adjusting a second on-time portion, TON2, of the ON-time duration inthe second drive cycle based on the duration of the delay time.
 2. Themethod as in claim 1 further comprising: adjusting the second on-timeportion such that a duration of the second on-time portion issubstantially proportional to the duration of the delay time.
 3. Themethod as in claim 1 further comprising: adjusting the second on-timeportion such that a duration of the second on-time portion issubstantially proportional to the duration of the delay time andsubstantially inversely proportional to a time duration since abeginning of the ON-time.
 4. The method as in claim 3 further comprisingmeasuring an auxiliary voltage across an auxiliary winding coupled tothe inductor; wherein driving the switch in a drive cyclecomprises:—detecting zero crossings of the auxiliary voltage and endingthe drive cycle after a predefined number of zero crossings hasoccurred; and wherein the predefined number of zero crossings isdependent on the feedback signal, and wherein obtaining the duration ofthe delay time comprises obtaining the predefined number.
 5. The methodas in claim 4 further comprising driving the switch when the auxiliaryvoltage has reached a local minimum magnitude.
 6. The method as in claim1, wherein the second ON-time portion occurs subsequent to the firstON-time portion.
 7. An apparatus comprising: a controller operative toexecute the method of claim
 1. 8. An apparatus comprising: a controlleroperative to: drive a switch, the switch coupled to an inductor in apower converter, the switch being driven during successive drive cycles,each of which includes an ON-time and an OFF-time of controlling theswitch; control a magnitude of an ON-time duration of the switch in asecond drive cycle depending on a feedback signal, the second drivecycle occurring subsequent to a first drive cycle of controlling theON-time duration of the switch; and wherein control of the magnitude ofthe ON-time duration in the second drive cycle comprises: i) adjustmentof a first on-time portion, TON1, of the ON-time duration in the seconddrive cycle based on the feedback signal, ii) determination of aduration of a delay time between a demagnetization time instance in thefirst drive cycle and an end of the first drive cycle, and iii)adjustment of a second on-time portion, TON2, of the ON-time duration inthe second drive cycle based on the duration of the delay time.
 9. Theapparatus as in claim 8, wherein the controller is further operative to:adjust the second on-time portion such that a duration of the secondon-time portion is substantially proportional to the duration of thedelay time.
 10. The apparatus as in claim 8, where in the controller isfurther operative to: adjust the second on-time portion such that aduration of the second on-time portion is substantially proportional tothe duration of the delay time and substantially inversely proportionalto a time duration with respect to a beginning of the on-time.
 11. Theapparatus as in claim 10, wherein the controller is further operativeto: measure an auxiliary voltage across an auxiliary winding coupled tothe inductor; detect zero crossings of the auxiliary voltage and endingthe drive cycle after a predefined number of zero crossings hasoccurred, wherein the predefined number of zero crossings is dependenton the feedback signal, and wherein obtaining the duration of the delaytime comprises obtaining the predefined number.
 12. The apparatus as inclaim 11, wherein the controller is further operative to: drive theswitch when the auxiliary voltage has reached a local minimum magnitude.13. The apparatus as in claim 8, wherein the second ON-time portionoccurs subsequent to the first ON-time portion.
 14. A method comprising:i) during a first control cycle: controlling operation of a switchcoupled to a primary winding of a transformer, the primary windinginductively coupled to a secondary winding of the transformer,activation of the switch storing energy in the transformer, the energyfrom the transformer producing output current from the secondary windingduring deactivation of the switch; and ii) during a second control cyclefollowing the first control cycle: controlling a magnitude of an ON-timeduration of activating the switch coupled to the primary winding of thetransformer based on a time delay between a first time instance in whichthe transformer becomes demagnetized in the first control cycle duringthe deactivation of the switch and a second time instance of activatingthe switch at a beginning of the second control cycle.
 15. The method asin claim 14 further comprising: supplying the output current from thesecondary winding to a rectifier operative to produce an output voltagebased on the output current.
 16. The method as in claim 14 furthercomprising: monitoring a magnitude of an output voltage derived from theoutput current; and controlling a magnitude of the time delay based onthe magnitude of the output voltage with respect to a reference voltage.17. The method as in claim 14, wherein a voltage across the primarywinding experiences parasitic oscillations during the time delay, themethod further comprising: monitoring the parasitic oscillations. 18.The method as in claim 17 further comprising: utilizing detected peaksor valleys of the parasitic oscillations to determine a start time inwhich to activate the switch in the second control cycle.
 19. The methodas in claim 14 further comprising: producing a first measurement valuecorresponding to a magnitude of voltage of the primary winding duringactivation of the switch in the first control cycle; producing a secondmeasurement value corresponding to the magnitude of voltage of theprimary winding during deactivation of the switch in the first controlcycle; and controlling the magnitude of the ON-time duration ofactivating the switch in the second control cycle based at least in parton the first measurement value and the second measurement value.
 20. Themethod as in claim 14 further comprising: monitoring an auxiliaryvoltage of an auxiliary winding inductively coupled to the primarywinding; monitoring occurrence of zero crossings associated with theauxiliary voltage; and terminating the first control cycle after apredefined number of zero crossings has been detected, the predefinednumber of zero crossings being dependent on a feedback signal, thefeedback signal generated based on a comparison of an output voltage anda reference voltage, the output voltage derived from the output current.